Polyurethane elastomer foams in a fine cellular structure have distinguished vibration damping property and shock absorbability, and are largely used as auxiliary springs, etc. in the automobile suspension section from the viewpoints of dynamic characteristics under a high load, durability and permanent set resistance. Particularly, polyurethane elastomer foams based on 1,5-naphthalene diisocyanate (NDI) have distinguished flex fatigue resistance characteristics, and thus are largely used in auxiliary spring positions, requiring high durability, However, NDI is higher in material cost than other diisocyanate compounds, and also is higher in the reactivity, that is, it has a hard-to-handle problem.
Breaking mechanism of automobile auxiliary springs seems to be due to heat generation of materials themselves caused by repeated flex transformations, resulting in a decrease in physical characteristics of materials exposed to the elevated temperature, and in ultimate generation of local cracking. Thus, it is indispensable that the auxiliary spring materials having a distinguished durability must undergo no more reduction in physical properties even in high-temperature circumstances, the polyurethane molecular chains having a strong cross-linking structure must furthermore have a higher level dimensional structure. However, the conventional processes such as butch mixing (one-shot method) of a diisocyanate with a high-molecular weight diol, and a chain extender (low-molecular weight diol), or preparation of prepolymer by reaction of a high-molecular weight diol with a diisocyanate in advance, followed by chain elongation by a low-molecular weight diol (prepolymer method) have been so far not successful in obtaining a higher level dimensional structure of polyurethane molecule chain having such a strong cross-linking structure from other aromatic diisocyanates than NDI.                Non-Patent Literature 1 Dictionary of Plastic Functional Polymeric Materials (2004), page 426, published by Dictionary Publishing Center of Sangyo-chosakai        